The study sheds fresh light on the well-understood problem of drive-by-downloads from compromised sites, a tactic that's come to eclipse virus-infected email as a means of spreading malware. Cybercrooks target users by spamvertising emails containing links to poisoned webpages, exposing unsuspecting victims to malware. At least one in ten web pages are booby-trapped with malware, according to a separate study by Google published last May.

Often these malware packages are designed to put compromised zombie PCs under the control of hackers.